Paragraph: When a metal rod M is dipped into an aqueous colourless...

Appearance of blue colour on addition of the metal rod $M$ to the solution of $N$ is an indication that metal may be $\mathrm{Cu}$. Later formation of white precipitate on addition of $\mathrm{NaCl}$ and dissolution of this precipitate in aqueous ammonia confirm that while preipitate is of $\mathrm{AgCl}$. This implies that the solution $N$ is of $\mathrm{AgNO}_3$. This confirm that the metal $M$ is only $\mathrm{Cu}$.